how about this.lets say i colonize 10,20, 200 systems and it turns out 50 of them where allready colonized by some entities, living in stars or gas giants.

like in The Saga of Seven Suns. you have a) the hydrogen who live in gas giants and b) the faeros(?) who live in suns and both are on an eternal war.humans didnt know about them until one point in their history.

to my question: will races who live in stars or gas giants or maybe even pure underground races on normal planets (basicly races who live on stellar bodies who are hard to scan through) be hard to see? meaning, will my ordinary scout ship discover such races?please say no

Something like this can be done only with "natives" on early tech level. Every scout ship in our solar system will detect human presence. It would be impossible to hide advanced race for long time, colonize their planet without knowledge about them.

So. Every race who lives on gas planets (Raefills), liquid (Olagarroans, Proteans) and stars (Stjornurans) will be detected by single scout ship.But your idea is very interesting if we will think about it as some kind of native race living deep under surface of such planets/stars